Recent trends in the design of disposable diapers have necessitated the development of more versatile adhesives for use in the diapers. The trend of the industry toward diapers with elastic bands on either the legs or the waist of the diaper have made it necessary to use a stronger more delamination resistant adhesive to hold the elastic in place. Most diaper manufacturers prefer to use one hot-melt adhesive to both construct the diaper (that is to bind the nonwoven top sheet and filler to the polyethylene back sheet) and to hold the elastic on the leg or waist of the diaper. It is well known that polyolefin based HMAs are suitable for the construction of diapers by construction techniques as currently practiced such as multiple fine line, hot-melt spray, hot-melt foam, slot coating operations, and various screen coating methods. However, polyolefin-based HMAs are traditionally not suitable for the bonding of the elastic to the diapers, that is, the elastic delamination resistance is insufficient for such an application. For this reason, adhesives based on styrene rubbers such as S-I-S block copolymers or S-B-S block copolymers are used (see, for example, U.S. Pat. No. 4,526,577).
The use of two HMAs on the same diaper poses some problems for the diaper manufacturer. They must insure the right adhesive is used in the right melt tank and is applied to the correct place on the diaper. Therefore, an adhesive that is capable of performing both types of bonding functions is highly desirable.
Wax is traditionally added to hot-melt adhesives with the objective of controlling viscosity of the adhesive. That is, it is a diluent and may in some cases lower the cost of the adhesive. For the most part waxes are not known to add to the strength of HMAs but rather are used to control set time or open time or viscosity.
This invention describes a polyolefin-based HMA suitable for use as both the adhesive of construction of disposable diapers and as the adhesive to hold the elastic onto the leg or waistband. Thus a universal diaper adhesive based on polyolefins has been developed. It has been surprisingly found that the addition of a low viscosity, substantially crystalline wax to a polyolefin-based HMA substantially improves the elastic delamination resistance. The formulation of an HMA containing an olefin-based polymer having sufficient elastic delamination resistance to meet the demands of the universal diaper adhesive is a significant advance in the art.